1. Field of the Invention
This invention relates generally to a current mirror circuit and, more particularly, is directed to a current mirror circuit for converting a single input current to an single output which is N times as much as the input current or for converting a single input current to a plurality of outputs the value of which can be chosen as desired.
2. Description of the Prior Art
Current mirror circuits, for example, for converting a single input current to an output current which is substantially equal to the input current are known in the art. Referring now to FIG. 1, there is shown one such current mirror circuit including a current supply source 1, a first transistor Q.sub.1 having its collector-emitter path connected between the current supply source 1 and a reference voltage source, a second transistor Q.sub.2 having its base and collector conntected together with the base of the first transistor Q.sub.1 and the emitter connected with the reference voltage source, and a third transistor Q.sub.3 having its base-emitter path connected in parallel with the collector-base path of the first transistor Q.sub.1 and the collector connected with an output terminal 2.
In this art, the base current i.sub.b3 of the third transistor Q.sub.3 is represented as follows: ##EQU1## where h.sub.fe =current gain of the transistors Q.sub.1 -Q.sub.3
i.sub.b =base current of the transistors Q.sub.1 and Q.sub.2
On the other hand, the input current I.sub.1 from the current supply source 1 is represented as follows: ##EQU2## Further, the output current I.sub.2 obtained from the collector of the transistor Q.sub.3 is represented as follows: ##EQU3## From the above equations (2) and (3), I.sub.2 is represented as follows: ##EQU4## From the equation (4), current gain I.sub.2 /I.sub.1 of this current mirror circuit is substantially equal to 1, regardless of the value of h.sub.fe.
Referring further to FIG. 2, a second known current mirror circuit is also proposed, in which the circuit construction is the same as the first known circuit except that the output current I.sub.2 is drawn from the collector of the transistor Q.sub.2 and the collector of the transistor Q.sub.3 is connected with the power supply source of +B.
From the above second circuit, the output current I.sub.2 is represented as follows in the same way as that of the first known circuit: ##EQU5## The equation (5) shows that the second known circuit is also adequate when the output current I.sub.2 is required to be substantially equal to the input current I.sub.1 regardless of the variation of h.sub.fe.
In the second known circuit, however, the output current I.sub.2 does not respond to the input current I.sub.1 when the input current varies in a high frequency, for example, 30 MH.sub.z by the effect of the storage delay-time of the transistors Q.sub.1 and Q.sub.2, because there is no current path that releases the base currents of transistors Q.sub.1 and Q.sub.2.
It is now realized from the above description that the high accuracy of I.sub.2 /I.sub.1 toward 1 is available from the prior art circuits. However, when an output current which is n times as much as the input current is required, the current gain I.sub.2 /I.sub.1 is not accurately equal to n in accordance with the construction of the above mentioned circuits.
Referring to FIG. 3, for example, if the emitter area of the transistor Q.sub.2 ' is formed twice as large as that of transistors Q.sub.1 and Q.sub.3, for example, the base current of the transistor Q.sub.2 ' is represented as 2i.sub.b and the base current i.sub.b3 of the transistor Q.sub.3 becomes as follows: ##EQU6## The input current I.sub.1 is represented as follows: ##EQU7## The output current I.sub.2 is represented as follows: ##EQU8## From the equations (7) and (8), the current gain .sup.I 2/I.sub.1 of this circuit is represented as follows: ##EQU9##
The equation (9) shows that coefficients of h.sub.fe are different between the denominator and the numerator, therefore the I.sub.2 /I.sub.1 is not accurately equal to 2. Especially, in case where the value of h.sub.fe is small or the frequency of the input current I.sub.1 is high, the current gain I.sub.2 /I.sub.1 becomes less accurate.
In another case, if a plurality of output currents, which have either different values or the same value, are required, the first known circuit is modified to have a plurality of output transistors. As shown in FIG. 4, for example if three different output currents I.sub.21, I.sub.22 and I.sub.23, the proportion of which to the input current I.sub.1 are p, q and r, respectively are required from the output terminals 21, 22 and 23, three output transistors Q.sub.31, Q.sub.32 and Q.sub.33, whose emitter areas are proportional to the emitter area of the transistor Q.sub.1 are p, q and r, respectively, for example, are needed and the emitter area of the transistor Q.sub.2 is to be formed (p+q+r) times as large as the emitter area of the transistor Q.sub.1.
In this case, the base current i.sub.b2 of the transistor Q.sub.2 is represented as follows: EQU b.sub.b2 =(p+q+r).multidot.i.sub.b ( 10)
Therefore, the sum of emitter currents of the transistors Q.sub.31, Q.sub.32, Q.sub.33 is represented as follows. EQU {(p+q+r).multidot.(h.sub.fe +1)+1}.multidot.i.sub.b ( 11)
Therefore, the sum of base currents of the transistors Q.sub.31, Q.sub.32, Q.sub.33 is represented as follows: ##EQU10## Therefore, the input current I.sub.1 is represented as follows. ##EQU11##
The output current I.sub.21 is obtained from the equation (6) as follows: ##EQU12##
From the equations (13) and (14) the output current I.sub.21 is further represented as follows: ##EQU13##
The other output currents I.sub.22 and I.sub.23 are obtained by substituting q and r for the coefficient p of the equation (15).
In the equation (15), however, coefficients of h.sub.fe are quite different between the denominator and the numerator, so that the current gain I.sub.21 /I.sub.1 is not p accurately. Therefore, from the circuit described above, plural number of output currents which have accurate current ratio to the input current are not available.